Ventricular fibrillation (VF) is the nnost common cause of sudden cardiac death (SCD), and accounts for over 300,000 deaths per year in the United States alone. However, despite 50 years of molecular and cellular research, no biological therapy has yet emerged with comparable efficacy to the implantable cardioverterdefibrillator. The objective of this proposed Program Project is to develop rational novel therapies to prevent SCD through a better understanding of the pathogenesis of VF at the mechanistic level. The proposal continues our efforts, which began with our SCOR in Sudden Cardiac Death (1995-2004) and has continued in the current Program Project (2005-2010), to address this objective by integrating information at the molecular, cellular, tissue and organism levels using a systems approach combining experimental and mathematical biology. Continuing along these lines, this Program Project will focus on trigger-substrate interactions, with the central theme related to how early (EADs) and delayed (DADs) afterdepolarizations, classically considered as arrhythmia triggers, simultaneously enhance the vulnerability ofthe tissue substrate to create the milieu leading to VF and SCD. We will analyze the synergism between dynamic factors and pre-existing tissue heterogeneities in this process. Project 1 (Multi-scale Modeling of Arrhythmias) wiil develop the theoretical framework, complemented by the experimental analysis at the molecular/cellular level in Project 2 (Cellular Mechanisms of Arrhythmias), the tissue level in Project 3 (Arrhythmias and Antiarrhythmic Targets in Failing Hearts), and therapeutic development in Project 4 (Molecular Approaches to Arrhythmia Therapy), facilitated by 3 cores (Computer and Math Core A, Biology and Bioengineering Core B, and Administrative Core C). Together, these studies will provide critical groundwork necessary to develop and advance novel therapies for this major complication and cause of mortality from heart disease.